A STUDY OF THE SKIN AND THE KIDNEYS 245 



on a house. A protective covering is thus formed which 

 does not hinder in any way the locomotion of the animal. 



Among the reptiles there are several types of scales. 

 Snakes are covered with a horny epidermis that is shed 

 usually in a single piece. In the rattlesnake a curious record 

 is kept of these inoultings, for when the rest of the skin is 

 dropped off, the two or three end joints are merely slipped 

 backward a little. Every moult, therefore, adds a new rattle 

 to the chain, but since this casting of the skin occurs at 

 irregular intervals, the string of rattles, even if complete, 

 does not, as is commonly supposed, indicate the age of the 

 snake. 



Crocodiles and alligators are incased in a covering of 

 bony scales which are more or less firmly attached to each 

 other, thus rendering locomotion on land slow and clumsy. 

 These scales are composed of both epidermis and dermis. 



One of the most perfect means of protection found in the 

 animal kingdom is the shell of the turtle. This is a box 

 minus the ends, into which the animal can withdraw its head, 

 legs, and tail. In order to understand this curious structure, 

 one must examine the upper shell from within. There the 

 vertebrae forming the spinal column become evident, but they 

 are immovably attached to the shell above, except in the 

 region of the neck and tail. The shell itself is composed 

 partly of the modified spinous and lateral processes of the 

 vertebrae, partly of bony plates formed in the dermis, and 

 both of these layers are covered over by the horny epidermis. 

 This last layer is the much-prized tortoise shell used in 

 making ornaments. 



The Skin of Birds. Feathers are the characteristic cover- 

 ing of birds, and they are the most wonderful of all skin 

 structures. In no other way has such a degree of lightness 

 been combined with such extent of surface and with such 

 strength. If we study the wing or tail feather of a chicken, 

 we find it to be constructed as follows. The hollow quill by 

 which the feather is attached to the skin of the bird is con- 



